Energy transfer of highly vibrationally excited molecules studied by crossed molecular beam/time-sliced velocity map ion imaging

The Good, the Bad, and the Ugly Memories of Carbohydrate Fragments in Collision-Induced Dissociation

Collision-induced dissociation (CID) tandem mass spectrometry is commonly used for carbohydrate structural determinations. In the CID tandem mass spectrometry approach, carbohydrates are dissociated into fragments, and this is followed by the structural identification of fragments through subsequent CID. The success of the structural analysis depends on the structural correlation of fragments before and after dissociation, that is, structural memory of fragments. Fragments that completely lose the memory of their original structures cannot be used for structural analysis. By contrast, fragments with extremely strong correlations between the structures before and after fragmentation retain the information on their original structures as well as have memories of their precursors' entire structures. The CID spectra of these fragments depend on their own structures and on the remaining parts of the precursor structures, making structural analysis impractical. For effective structural analysis, the fragments produced from a precursor must have good structural memory, meaning that the structures of these fragments retain their original structure, and they must not be strongly affected by the remaining parts of the precursors. In this study, we found that most of the carbohydrate fragments produced by low-energy CID have good memory in terms of linkage position and anomericity. Fragments with ugly memory, where fragment structures change with the remaining parts of the precursors, can be attributed to C ion formation in a linear form. Fragments with ugly memory can be changed to have good memory by preventing linear C ion generation by using an alternative CID sequence, or the fragments of ugly memory can become useful in structural analysis when the contribution of linear C ions in fragmentation patterns is understood.

Electrospray ionization in-source decay of N-glycans and the effects on N-glycan structural identification

RATIONAL

Electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are soft ionization techniques commonly used in mass spectrometry. Although in-source and post-source decays of MALDI have been investigated extensively, the analogous decays of ESI have received little attention. Previous studies regarding the analogous decays of ESI focus on the dissociation of multiply charged proteins and peptides. The decay of carbohydrates in ESI has not been investigated yet, and it may have interference in carbohydrate structural determination.

METHODS

Commercial apparatus, including a high-performance liquid chromatography (HPLC), an ESI source, and a linear ion trap mass spectrometer, were used to investigate the fragmentation of several N-glycans during the ESI process.

RESULTS

About 0.2%-3% of neutral N-glycans and more than 50% of N-glycans consisting of a sialic acid are dissociated into small N-glycans by ESI in-source decay in typical ESI operating conditions. The efficiencies of most dissociation channels increase as the temperature of ion transfer capillary increases, indicating that part of the energy deposited into the precursor ions for cracking is from the heated capillary. The cracking patterns of ESI in-source decay are slightly different from those of gaseous phase collision-induced dissociation.

CONCLUSIONS

Large N-glycans are dissociated into small N-glycans in ESI in-source decay that may result in the interference of the structural identification of small N-glycans. Separation of large N-glycans from small N-glycans, for example, using HPLC, prior to ESI ionization is necessary to eliminate the interference. This is particularly important when N-glycans consist of sialic acid or large N-glycans have much higher concentration than that of small N-glycans in ESI solution.

Collision-Induced Dissociation of Cellobiose and Maltose

Structure determination is a longstanding bottleneck of carbohydrate research. Tandem mass spectrometry (MS/MS) is one of the most widely used methods for carbohydrate structure determination. However, the effectiveness of MS/MS depends on how the precursor structures are derived from the observed fragments. Understanding the dissociation mechanisms is crucial for MS/MS-based structure determination. Herein, we investigate the collision-induced dissociation mechanism of β-cellobiose and β-maltose sodium adducts using quantum chemical calculations and experimental measurements. Four dissociation channels are studied. Dehydration mainly occurs through the transfer of an H atom to O1 of the sugar at the reducing end, followed by a C1-O1 bond cleavage; cross-ring dissociation starts with a ring-opening reaction, which occurs through the transfer of an H atom from O1 to O5 of the sugar at the reducing end. These two dissociation channels are analogous to that of glucose monosaccharide. The third channel, generation of B₁ and Y₁ ions, occurs through the transfer of an H atom from O3 (cellobiose) or O2 (maltose) to O1 of the sugar at the nonreducing end, followed by a glycosidic bond cleavage. The fourth channel, C₁-Z₁ fragmentation, has two mechanisms: (1) the transfer of an H atom from O3 or O2 to O4 of the sugar at the reducing end to generate C ions in the ring form and (2) the transfer of an H atom from O3 of the sugar at the reducing end to O5 of the sugar at the nonreducing end to produce C ions in the linear form. The results of calculations are supported by experimental collision-induced dissociation spectral measurements.

Collision-induced dissociation of xylose and its applications in linkage and anomericity identification

Different dehydration barrier heights result in different branching ratio, a simple and fast anomeric configuration identification for xylose.

Theoretical study of the complexes of dichlorobenzene isomers with argon. I. Global potential energy surface for all the isomers with application to intermolecular vibrations

The complexes of para- (p-), meta- (m-), and ortho- (o-)dichlorobenzene (DCB) isomers with argon are studied using an ab initio method. The interaction energy in the ground electronic state of the complexes has been calculated using the CCSD(T) method (coupled cluster method including single and double excitations with perturbative triple excitations) and Dunning's double-ζ (aug-cc-pVDZ) basis set supplemented by midbond functions. Local interaction parameters have been defined and interesting relations fulfilled by them, independent of the DCB isomer, have been revealed. This finding has allowed us to construct the accurate global analytical intermolecular potential energy surface for all the DCB-Ar complexes with the same set of parameters, except for the monomer geometries. Each complex is characterized by two symmetrically equivalent global minima, one located above and the other located below the monomer plane at distances equal to 3.497 Å, 3.494 Å, and 3.485 Å for p-, m-, and o-isomers of DCB bound to Ar, respectively. Additionally, the Ar atom is shifted from the geometrical center of the DCB monomer towards the chlorine atoms by the value x_e of 0.182 Å for m-isomer and 0.458 Å for o-isomer. The calculated binding energy D_e of 460 cm^-1, 465 cm^-1, and 478 cm^-1 for p-, m-, and o-complex, respectively, are related to x_e by simple relations. The intermolecular bending fundamentals calculated from PES depend strongly on the isomer structure. The calculated dissociation energies fit in the intervals estimated by the experiment of Gaber et al. for the S₀ state [Phys. Chem. Chem. Phys. 11, 1628 (2009)].

Collision-induced dissociation of sodiated glucose, galactose, and mannose, and the identification of anomeric configurations

Different dehydration barrier heights of cis and trans configurations between O1 and O2 provide a simple and fast anomeric configuration identification.

Does low-energy collision-induced dissociation of lithiated and sodiated carbohydrates always occur at anomeric carbon of the reducing end?

RATIONALE

Collision-induced dissociation (CID) tandem mass spectrometry is one of the major methods in the structural determination of carbohydrates. Previous experimental studies and theoretical investigation of lithiated and sodiated underivatized carbohydrates seem to indicate that dehydration reactions and cross-ring dissociation of low-energy CID mainly occur at the anomeric carbon of the reducing end. However, these studies only investigated a few carbohydrates.

METHODS

ESI-MS/MS spectra of [M + Li]⁺ and [M + Na]⁺ ions of several ¹⁸ O1-labeled monosaccharides and disaccharides at O1 of the reducing end were studied using a linear ion trap mass spectrometer.

RESULTS

Dissociations from the losses of both labeled and unlabeled neutral fragments were observed. The branching ratios of dissociations from the losses of unlabeled neutrals for dehydration reactions are larger than that for cross-ring dissociation; lithiated carbohydrates are larger than sodiated carbohydrates, and 1-4 linkages of disaccharides are larger than the other linkages. For some lithiated carbohydrates, dehydration reactions from the losses of unlabeled neutrals have larger branching ratios than that from the losses of labeled neutrals. The fragments from the losses of unlabeled neutrals investigated using MS³ showed that the losses of unlabeled H₂ O mainly occur at the reducing monomer for sodiated carbohydrates, but the losses of unlabeled C₂ H₄ O₂ for lithiated carbohydrates can occur at both reducing and nonreducing monomers. The ratio of B₁ and Y₁ ions to C₁ and Z₁ ions of disaccharides is related to the cis or trans configuration of the O1 and O2 atoms in the nonreducing monomer. The results are explained by the properties of transition states of dissociation channels.

CONCLUSIONS

Our data shows that dehydration reactions and cross ring dissociation do not always occur at the anomeric carbon atom of the reducing monomer.

Collision-induced dissociation of sodiated glucose and identification of anomeric configuration

Difference in dehydration barrier heights results in different branching ratio, a simple and fast method for anomeric configuration identification.

Ion-to-Neutral Ratios and Thermal Proton Transfer in Matrix-Assisted Laser Desorption/Ionization

The ion-to-neutral ratios of four commonly used solid matrices, α-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (2,5-DHB), sinapinic acid (SA), and ferulic acid (FA) in matrix-assisted laser desorption/ionization (MALDI) at 355 nm are reported. Ions are measured using a time-of-flight mass spectrometer combined with a time-sliced ion imaging detector. Neutrals are measured using a rotatable quadrupole mass spectrometer. The ion-to-neutral ratios of CHCA are three orders of magnitude larger than those of the other matrices at the same laser fluence. The ion-to-neutral ratios predicted using the thermal proton transfer model are similar to the experimental measurements, indicating that thermal proton transfer reactions play a major role in generating ions in ultraviolet-MALDI.

Ion-to-neutral ratio of 2,5-dihydroxybenzoic acid in matrix-assisted laser desorption/ionization

RATIONALE

In most previous studies, the ratios of desorbed ions and neutrals from matrix-assisted laser desorption/ionization (MALDI) were measured outside the common MALDI conditions. In this work, we measured the ratios under common MALDI conditions.

METHODS

Ions were detected using a time-of-flight mass spectrometer in combination with a time-gated ion imaging detector. Mass-resolved desorbed neutral molecules at different angles and velocities were measured using a modified crossed molecular beam apparatus.

RESULTS

The upper limit of the ion-to-neutral ratio from pure 2,5-dihydroxybenzoic acid (25DHB) is 4 × 10(-9) at laser fluence 40 J/m(2), it increases to 3 × 10(-7) at laser fluence 250 J/m(2). The ratios of matrix from the mixture of 25DHB and analyte remain in the same order of magnitude as pure 25DHB. However, the ratio of analyte depends strongly on the analyte. Values as large as 10(-3)-10(-4) for bradykinin and as small as <10(-8) for glycine were observed at laser fluence ~100 J/m(2).

CONCLUSION

The ion-to-neutral ratios of 25DHB matrix measured in this work are much smaller than some of the values reported in previous work using different methods and/or under different MALDI conditions.